1. Field of the Invention
The present invention relates to a high frequency oscillation type proximity sensor and, more particularly, to a proximity sensor having a feature in that a positioning structure for a coil spool and a core is characteristic. The present invention also relates to an assembling method of the coil spool and the core of the proximity sensor.
2. Description of the Related Art
In a high frequency oscillation type proximity switch, it is necessary to set deviations in detection characteristic, typically represented by the detection distance, between products to a value not more than a specific value. In order to make the deviations in detection characteristic smaller, in most cases, circuit adjustments are carried out by using a variable resistor or the like during the assembling processes of the product; however, by making deviations in the positions of the members on the periphery of the detection coil such as a coil spool, a core and a coil case smaller, it is possible to narrow the adjusting range of the circuit or to eliminate the adjustments of the circuit.
The core is formed by sintering ferrite powder that has been molded, and since the powder shrinks by 10 to 20% during the sintering, it is difficult to improve the dimension precision. Therefore, in the case when positioning is carried out by using a simple fitting structure between the core and the coil spool or between the core and the coil case, since it is necessary to provide a gap-fitting structure with a margin, it is not possible to achieve positioning between members on the periphery of the detection coil with high accuracy. In addition, with respect to the members on the periphery of the detection coil, it is desirable not only to simply provide a positioning process with high accuracy, but also to provide a preferable assembling workability. The following documents relate to the positioning and the assembling workability of the members on the periphery of the detection coil of a proximity switch.
In the proximity switch disclosed in Japanese Unexamined Utility Model Publication No. 64-21939, in order to ensure the mechanical coupling between a bobbin (coil spool) and a core and also to provide an attaching device that achieves excellent workability and makes deformation of the bobbin smaller, an elastic bridge portion is formed on the cylinder portion of the bobbin, and a protrusion that is elastically brought into contact with the center portion of the core is formed at the intermediate portion of the bridge portion. However, in the embodiment shown in the figure, since two-point contacts are made between the protrusion of the bridge portion and the center portion of the core, it is not possible to ensure a coaxial structure between the bobbin and the core, and the bobbin axis tends to tilt from the core axis. Moreover, no consideration is given to the positioning in the axial direction with high accuracy.
With respect to the assembling method for a proximity switch, Japanese Unexamined Utility Model Publication No. 3-99305 has disclosed a method in which the flange face of a coil bobbin is placed on a flat plate and this is covered with a ferrite magnetic core (core), so that the flange face of the coil bobbin and the end face of the ferrite magnetic core are placed onto the same face. In this method, however, it is not possible to ensure the coaxial property between the coil bobbin and the ferrite magnetic core, and the assembling uses a bonding agent that is placed with a volume, and cured, thereby failing to provide a method with high productivity.
In the proximity switch disclosed in Japanese Unexamined Utility Model Publication No. 1-152431, an extended portion that is extended in the diameter direction is formed on the flange of the detection end side of the coil spool, and a step portion that is the same as the thickness of the extended portion is formed on the end face of the core side leg portion (circumferential wall portion); thus, the end face of the detection-end side of the coil spool and the end face of the core side leg portion are positioned on the same face. In this arrangement, however, it is not possible to ensure the coaxial property of the coil spool and the core as well as the coaxial property of the core and the coil case.
In none of the above-mentioned conventional techniques, it is not possible to simultaneously ensure the mutual coaxial property of the core, coil spool and coil case and the positioning in the axial direction thereof with sufficiently high accuracy.
It is an object of the present invention to provide: a high-frequency oscillation type proximity sensor having small deviations among products in detection property and having excellent assembling workability because of small mutual positional deviations of at least a core and coil spool, more preferably, small mutual positional deviations among members including a coil case in addition to these; a detection end module used therefor; and an assembling method of a coil spool a core used therefor.
A proximity sensor according to the present invention includes: a detection coil; a detection circuit including an oscillation circuit having the detection coil as a resonance circuit element; a coil spool including a hollow cylindrical wire-winding drum around which a lead wire of the detection coil is wound, a detection-side flange provided on a detection side end of the wire-winding drum, and a circuit side flange provided on the detection circuit side end of the wire-winding drum; a core, made of a magnetic member, including a shaft inserted into the wire-winding drum of the coil spool, and a plate-shaped base located between the coil spool and the detection circuit; a cylindrical coil case housing the coil spool and the core and having a bottom on the detection side end; and a main body case, fitted to the coil case, for housing the detection circuit, wherein the wire-winding drum of the coil spool includes protrusions that protrude toward the center from not less than three points distributed in the range exceeding a semi-circle on the circumference of the wire-winding drum and that are elastically brought into contact with a shaft of the core, respectively, thereby coaxially positioning the coil spool and the shaft of the core.
According to the present invention, the elastic contact between the protrusion of the coil spool and the shaft of the core makes it possible to coaxially position the coil spool and the core. It is preferable to position all the coil spool, core and the main body case (or the coil case to be fitted to the main body case) mutually without deviations; however, since the coaxial property of the coil spool and the core gives more influences to the detection property of the proximity sensor rather than to the positional relationship between these and the main body case, even achieving only the coaxial property makes it possible to limit deviations in detection characteristics among the products to a considerably small range. In addition, even combining only the coil spool and the core through an elastic contact makes it possible to achieve the coaxial property, and this position-determined state is maintained even until the time at which it has been finally cured by resin; therefore, it becomes possible to eliminate the coating process of an adhesive agent and the stand-by process for the curing so as to maintain the position-determined state, and consequently to provide a superior assembling workability.
In addition to a proximity switch for outputting a binary signal corresponding to the presence or absence of object detection, the proximity sensor herein includes of which outputs an analog signal corresponding to the intensity of the object detection and a digital signal formed by encoding this signal, and those which outputs the resulting signal obtained by subjecting these to any signal treatment. The intensity of the object detection varies when any of the distance to an object, the size of the object and the material of the object vary.
This proximity sensor preferably has a shape that applies a force to the shaft of the core at positions that are dispersed along the axial direction.
Herein, the application of a force to the shaft of the core at positions that are dispersed along the axial direction refers to a case where the positions of the core shaft at which the force is applied are dispersed at such positions that can prevent the tilt of the core shaft to the coil spool even when an external force is applied, and includes a case where the number of positions at which the force is applied is not less than two in addition to a case where the entire portion having a certain length along the axial direction of the protrusion applies a force to the core shaft. Here, the positions at which a force is applied may be located at different levels by the protrusion. Another arrangement may be formed in which, although the position at which any of the protrusions apply a force is only one position, when protrusions of not less than three are viewed as a whole, a force is applied to the shaft at positions of not less than two in the axial direction.
In this proximity sensor, the core may further include a circumferential wall covering the outer periphery of the detection coil, and the detection-side flange of the coil spool and the inner face of the cylinder portion of the coil case may be fitted to each other.
The proximity sensor, provided with the circumferential wall portion in the core, is referred to as a sealed type. By fitting these to each other, the coil spool and the coil case are positioned with respect to the center axis and the vertical direction. The expression, xe2x80x9cfitted to each otherxe2x80x9d, refers to a state where the gap is so small that the positioning is achieved or a state where two members are fitted to each other without a gap. Since the coaxial property between the coil case and the core has already been achieved, the positioning among the core, the coil case and the main body case is also achieved without deviations in the axis and the vertical direction.
When the detection-side flange of the coil spool can be positioned through a fitting operation to the coil case, it is sufficient for a portion thereof to be associated with the fitting operation. When the tubular portion of the coil case has a cylindrical shape, the detection-side flange may be set to a round shape or a shape other than the round shape. In the case when the detection-side flange does not have a round shape, a portion that is coincident with the circumscribing circle of the detection-side flange is fitted to the cylinder shape portion of the coil case. In this case, the portion of the detection-side flange relating to the fitting operation is distributed to not less than three positions covering a range exceeding the semicircle thereof.
Such a portion relating to the fitting operation is exemplified by an arm installed in a manner so as to stick out from the outer circumferential portion of the detection-side flange of the coil spool. In this case, the circumferential wall portion of the core is designed to have a concave portion having a depth corresponding to the thickness of the arm with the arm passing through the concave portion.
This concave portion may be used as a positioning reference in the core axial direction with respect to the coil spool, or a stopper which prevents the positioning between the coil spool and the core that have the respective detection-side end faces as the identical face from offsetting greatly.
In another aspect of the proximity sensor of this invention, the circuit-side flange of the coil spool is fitted to the inner face of the tubular portion of the coil case, and the detection-side flange of the coil spool can be set to a gap-fitting state with a margin to the inner face of the tubular portion of the coil case.
In this case, the proximity sensor is referred to as a non-sealed type without the circumferential wall in the core. In this type, when the main body case is made of metal, a coil case made of resin is generally allowed to protrude from the main body case with no metal case being located on the side face of the coil. Since there is no circumferential wall in the core, it is possible to fit the circuit-side flange of the coil spool to the coil case. In this case, provision may be made so as to also fit the detection-side flange of the coil spool to the coil case; however, this arrangement makes the resistance greater when the coil spool is inserted into the coil case. Therefore, the detection-side flange is formed into a shape and a size that provide a sufficient gap to the coil case so that it becomes possible to easily insert the coil spool into the coil case, and consequently to improve the assembling workability.
In the proximity sensor in this case, it is preferable to allow a portion of the outer circumference of the circuit-side flange to devote to the fitting operation to the coil case. Thus, it is possible to provide a sufficient gap to the coil case inner face in the other portion of the outer circumference of the circuit-side flange so that resin to be injected to the coil case is allowed to flow into the detection coil side through this gap.
A detection-end module for use in a proximity sensor according to the present invention includes: a detection coil; a detection circuit assembly including an oscillation circuit having the detection coil as a resonance circuit element; a coil spool including a hollow cylindrical wire-winding drum around which a lead wire of the detection coil is wound, a detection-side flange provided on a detection side end of the wire-winding drum, and a circuit side flange provided on the detection circuit side end of the wire-winding drum; and a core, made of a magnetic member, including a shaft inserted into the wire-winding drum of the coil spool, and a plate-shaped base located between the coil spool and the detection circuit; a cylindrical coil case housing the coil spool and the core and having a bottom on the detection side end, the members being integrally formed therein. The wire-winding drum of the coil spool includes protrusions that protrude toward the center from not less than three points distributed in the range exceeding a semi-circle on the circumference of the wire-winding drum and that are elastically brought into contact with a shaft of the core, respectively, thereby coaxially positioning the coil spool and the shaft of the core. The detection circuit assembly is designed so as to externally output a signal having a predetermined shape corresponding to the oscillation state of the oscillation circuit as an object detection signal of the proximity sensor.
According to this detection end module, since the coaxial property between the coil spool and the core is high, the necessity of adjustments in detection characteristics of the circuit becomes smaller correspondingly, and the application of this makes it possible to simplify the circuit structure of the proximity sensor and also to improve the productivity thereof. The detection end module of this type can be preferably put into the market independently as a part to be used for manufacturing the proximity sensor.
A coil spool and a core to be used in the proximity sensor according to the present invention can be assembled through the following steps by using a mounting base having a reference face and a pilot pin capable of reciprocally moving through the mounting base. In a first step, the pilot pin protruding from the mounting base is inserted into the wire-winding drum of the coil spool in such a manner as to direct the detection-side flange of the coil spool toward the reference face of the mounting base. In a second step, the pilot pin is pulled out from the coil spool while the detection-side flange of the coil spool is brought into contact with the reference face of the mounting base and, also, the shaft of the core is inserted into the coil spool in place of the pilot pin. In a third process, a relative position of the coil spool and the core is determined by bringing the detection-side end face of the core into contact with the reference face of the mounting base.
According to this method, the assembling can be carried out by easily aligning the end face of the coil spool and the end face of the core on the same plane. In addition, the application of the pilot pin makes it possible to easily insert the shaft of the core into the coil spool without causing any chipping or abrasion in the core shaft.